Systems for providing electrical stimulation of the brain are increasingly popular for the treatment of neurological disorders such as epilepsy or migraine headaches. A brain neurostimulator device may include a signal processor to detect the onset of a neurological event and a pulse generator for providing electrical stimuli to treat the neurological event. The neurostimulator device further includes one or more leads having one or more electrodes operatively connected to the brain. The neurostimulator device sends and receives electrical signals to and from the brain via the electrodes on the lead. Access to the brain is achieved by drilling a hole in the patient's skull. The lead is passed through the hole and positioned at the desired site or sites on or within the brain. The lead is typically secured to the skull by an anchoring system and the signal processor and pulse generator portions of the neurostimulator device generally reside outside the patient's cranium.
Although the signal processing and pulse generator portions of neurostimulator devices are often not implanted in the cranium, many functional and aesthetic advantages can be gained in doing so. For example, as can be appreciated, many opportunities exist for traction on the portion of the lead that is positioned outside the skull in systems where the signal processing and pulse generator portions of neurostimulator devices reside outside the cranium. In such systems, the leads need to be tunneled along the neck and down the pectoral region where the neurostimulator resides, for example, as in the Medtronic Activa® device of Medtronic, Inc., Minneapolis, Minn. Such an implementation subjects the leads to fatigue. Any accidental tug on the lead outside of the skull could result in movement of the portion of the lead positioned within the skull. If the lead is accidentally moved such that the electrodes are also dislocated from their sites, the continued proper functioning of the neurostimulator device would require invasive repositioning of the electrodes. In current designs, the success of preventing the lead from being moved depends upon the effectiveness of the lead anchoring system. Implanting the neurostimulator within the cranium of the patient would help reduce traction on the leads.
One example of an implantable system is disclosed in U.S. Pat. No. 6,016,449 entitled “System for Treatment of Neurological Disorders” issued to Fischell, et al. on Jan. 18, 2000 (hereinafter “Fischell”). Fischell discloses a responsive detection and stimulation system for the early recognition and prompt treatment of a neurological event arising from neurological disorders such as epilepsy, migraine headaches, and Parkinson's disease. In Fischell, the entire implantable portion of the system for treating neurological disorders lies under the patient's scalp. By placing the entire system within the cranium, as opposed to having some wires extending into or through the neck to a control module in the chest, the probability of wire breakage due to repeated wire bending is drastically reduced. Other examples of devices implanted in the cranium for applying electrical stimulation therapies to electrodes sited at appropriate locations include cochlear implants.
Implantation of a neurostimulator device into the skull is complicated by a number of factors. For example, implantation requires surgical removal of a portion of the cranium that is generally larger than that required for the implantation of anchoring systems for securing a lead alone. Further, skull dimensions, contours and thicknesses vary from patient to patient. Nevertheless, a standard-shaped device is desirable, as is a relatively low-cost means for adapting a standard-shaped device to the confines of the cranial opening unique to each patient.
One solution, proposed in this patent, for adapting an implantable device to the confines of the cranial opening is a ferrule. The ferrule, which is secured to the cranium, is designed to receive and secure a neurostimulator device. The ferrule has an inner shape that is adapted for receiving and securing neurostimulator devices and an outer shape that substantially conforms to the unique cranial opening of the patient. In essence, the ferrule helps de-couple the device shape from the shape of the cranial opening.
Additional functional demands of the ferrule include supporting and preventing the neurostimulator device from severely impinging on the brain as a result of impact to the device. Also, the neurostimulator device must be secured to the ferrule to prevent its accidental dislocation, yet allow for its removal or access for maintenance or the replacement of the battery or device. Furthermore, the ferrule must be adequately secured to the cranium of the patient and aesthetically blend with the exterior contour of the skull. The present invention is directed to satisfying these needs.